Altruism among family members can inhibit cooperation by increasing the utility players expect to receive in a non-cooperative equilibrium. To test this, we examine agricultural productivity in West African polygynous households. We find cooperation, as evidenced by more efficient production, is greater among co-wives than among husbands and wives. Using a game-theoretic model, we show this outcome arises because co-wives are less altruistic towards each other than towards their husbands. We present a variety of robustness checks, which suggest results are not driven by selection into polygyny, greater propensity for cooperation among women, or household heads enforcing others’ cooperative agreements.